1. Field of the Invention
The present invention generally relates to signal transmission lines and more particularly to adaptive control of the capacitance (and delay) of transmission lines.
2. Description of the Related Art
The propagation characteristics of transmission lines within high-speed digital devices and integrated circuits often need to be adjusted to ensure proper operation of the devices. For example, within integrated circuits, it is often necessary to synchronize clock signals between transmission lines which have fundamentally different delay characteristics. The delay characteristics of transmission lines can be controlled by altering the capacitance of the transmission line. Therefore, there is a need for an electrically variable transmission line on a semiconductor chip which is useful in analog functions, data processing, memory, and telecommunications applications.
Conventionally, the capacitance of a transmission line can be altered by including inductors, capacitors and resistors either in parallel or in series with the transmission line. However, such devices do not offer the ability to dynamically alter the capacitance of the transmission line because the capacitance of the transmission line is permanently changed once such devices are attached to the transmission line.
Additional structures have been designed to dynamically vary the capacitance of a transmission line. In one such device, the capacitance (and delay characteristics) of the transmission line is dynamically changed by connecting capacitive elements such as varactor diodes in parallel with the transmission line. In such a structure, the diodes are selectively energize to add or subtract capacitance to or from the transmission line.
It is, therefore, an object of the present invention to provide a semiconductor structure having a substrate, an insulator above a portion of the substrate, a conductor above the insulator; and at least two contact regions in the substrate, with at least one of the contacts shared with other similar semiconductor structures, such that a voltage between the contact regions modulates a capacitance of the conductor.
The substrate includes an inversion region below the insulator and a depletion region below the inversion region, wherein the voltage modulates a capacitance of the inversion region. The capacitance of the conductor is equal to the capacitance of the inversion region in series with a capacitance of the insulator.
One of the contact regions is a region of the substrate doped with a first impurity and another of the contact regions is a region of the substrate doped with a second impurity opposite the first impurity.
The contact regions can include contact conductors in trenches adjacent the portion of the substrate, the portion of the substrate includes first impurity regions adjacent the trenches and a second impurity region between the first impurity regions, wherein the first impurity is an opposite type from the second impurity, and the second impurity region is a depletion region below the insulator. Boundaries of the depletion region are controlled by the voltage applied to the contact conductors. Additionally, the conductor can be a plurality of conductors above the portion of the substrate and the voltage applied to the contact conductors independently varies a capacitance of different portions of the depletion region. An important feature of the invention is that the capacitance of the conductor is independent of the voltage of the conductor.
Another embodiment of the invention is a variable capacitance transmission line structure including a substrate, an insulator above a portion of the substrate, a transmission line above the insulator, at least two contact regions in the substrate (such that a voltage between the contact regions modulates a capacitance of the transmission line) and a delay control circuit supplying the voltage to the contact regions.
The substrate includes an inversion region below the insulator and a depletion region below the inversion region, wherein the voltage modulates a capacitance of the inversion region. The capacitance of the transmission line is equal to the capacitance of the inversion region in series with a capacitance of the insulator.
One of the contact regions is a region of the substrate doped with a first impurity and another of the contact regions is a region of the substrate doped with a second impurity opposite the first impurity. At least one contact region is shared with more than one structure. The contact regions include contact conductors in trenches adjacent the portion of the substrate, the portion of the substrate includes first impurity regions adjacent the trenches and a second impurity region between the first impurity regions, wherein the first impurity is an opposite type from the second impurity; and the second impurity region is a depletion region below the insulator. Boundaries of the depletion region are controlled by the voltage applied to the contact conductors. Also, the transmission line can be a plurality of transmission lines above the portion of the substrate and the voltage applied to the contact conductors independently varies a capacitance of different portions of the depletion region. As mentioned above, an important feature is that the capacitance of the transmission line is independent of the voltage of the transmission line.
Another embodiment of the invention is a three terminal varactor that includes a substrate, an insulator above a portion of the substrate, a first contact above the insulator, a second contact in the substrate, a third contact in the substrate, such that a voltage between the second contact and the third contact modulates a capacitance between the first contact and the second contact.
The substrate includes an inversion region below the insulator and a depletion region below the inversion region, wherein the voltage modulates a capacitance of the inversion region. The capacitance between the first contact and the second contact is equal to the capacitance of the inversion region in series with a capacitance of the insulator. The second contact is a region of the substrate doped with a first impurity and the third contact is a region of the substrate doped with a second impurity opposite the first impurity. With the inventive three terminal varactor, the capacitance of the conductor is independent of the voltage of the conductor.
In another embodiment the invention comprises an integrated circuit chip that includes a signal line carrying signals and having a delay associated with it, at least one charge region having a controllable charge density near the signal line, and a modulation circuit coupled to the charge region for dynamically controlling the charge density in the charge region for adjusting the delay associated with the signal line.
The charge region includes an inversion region adjacent the signal line, wherein the modulation circuit modulates a capacitance of the inversion region.
The capacitance of the signal line is equal to the capacitance of the inversion region in series with a capacitance of an insulator adjacent the signal line. Further, the capacitance of the signal line is independent of a voltage of the signal line.
The invention also includes a three terminal varactor comprising a silicon body region, an anode terminal coupled to the silicon body region, a receiving terminal for receiving a signal, for receiving a bias, and having a capacitance between itself and the anode terminal (the bias for controlling a capacitance range between the receiving terminal and the anode terminal), and a control terminal coupled to the silicon body region for adjusting the capacitance within the capacitance range. The capacitance of the receiving terminal is independent of the bias.